The goal of the present project is to investigate the structural aspect force generation in skeletal muscle by studying X-ray diffraction patterns from single muscle fibers under a wide range of conditions. A central question is what structural changes occur among the crossbridges (myosin) attached to actin associated with force generation. In previous years we have investigated the mass distribution of the attached crossbridges in relaxed and rigor states. Last year we obtained equatorial diffraction patterns from fully activated muscle cells at low ionic strength by using the synchrotron X-ray source in Hamburg, FRG. By comparing diffraction patterns from fibers in the relaxed, and the fully Ca++ activated, and the rigor states in the same preparation, structural changes involved in force generation may be determined. Preliminary analysis of the results shows that upon activation the intensity of the first reflection on the equator (I10) decreased by 15% while the second reflection remains unchanged to within experimental error. Assuming that attached crossbridges in relaxed fibers are in a state prior to force generation in the crossbridge cycle, the present results show that the force generating process, the "power stroke", involves a configurational change sufficiently large that it is detectable by equatorial X-ray diffraction at 228 angstrom resolution. Interpretation of electron density maps of the muscle cells based on the X-ray diffraction data is complicated by the lack of phase information and the effects of limited resolution. To ensure proper interpretation, systematic model calculations were carried out to determine correlations between mass distributions on one hand, and phases and amplitudes on the other. One of the major findings is that the radial position of the center of mass of the myosin heads has strong effects on the phases and amplitudes of the reflections on the equator.